1. Field of the Invention
This invention relates to a mask used for an exposure process, and more particularly, to a mask used for an exposure process using x-ray.
2. Description of Related Art
In a photolithography process, in addition to a light source, a mask with a pattern of circuits, for transferring the pattern onto a photoresist layer, is also required. Since a whole semiconductor wafer is wasted if any single processing step goes wrong, inspection steps are required before proceeding with any fabrication step. As a result, it is very important for the fabrication of semiconductor devices to correct the alignment of the layers before every photolithography process to avoid improperly transferring the patterns.
Referring to FIG. 1A through FIG. 1E, a fabrication process of a conventional mask used in a x-ray lithography mask process is shown.
First, as shown in FIG. 1A, two membranes 3 and 4, such as silicon-rich nitride, are formed on the first surface and the second surface of an experimental silicon wafer 2. Then, referring to FIG. 1B, an etching process, such as an etching process using KOH, is performed on the middle portion of membrane 4 and on wafer 2 itself. The etching process is stopped when membrane 3 is exposed.
Referring to FIG. 1C and FIG. 1D, an absorber 6 is formed on the membrane 3. Next, photoresist layer 8 is deposited on the absorber 6 and patterned, and the unwanted portions of the absorber are etched off. Then, a photolithography process is performed to transfer a desired pattern onto the absorber 6.
Referring to FIG. 1E, after the removal of the patterned photoresist 8, an anti-reflection layer 10 is then deposited on the patterned absorber 6, and an x-ray mask is completed. The anti-reflection layer 10 prevents the transfer of patterns caused by exposure to secondary electrons due to excessive x-ray energy.
Because the absorber 6 is capable of absorbing high-energy x-rays, the density of the absorber is relatively high. As a result, the selective etching process on the membrane 3, which has a thickness of only about lam, and the absorber 6 is accomplished at a very low yield, and the fabrication cost is increased accordingly.
Furthermore, after several exposures to x-rays, a thermal stress occurs on the transferred pattern of the absorber 6, and twists the membrane 3, causing misalignment. As a result, patterns cannot be reliably or precisely transferred by photolithography processes.
The fabrication process of another conventional mask for x-ray is shown in FIG. 2A through FIG. 2E.
First, referring to FIG. 2A and FIG. 2B, membranes 3 and 4, such as silicon-rich nitride, are formed respectively on the first surface and the second surface of an experimental silicon wafer. An absorber 6 is then formed on the upper membrane 3.
Referring to FIG. 2C, a photoresist layer 8 is deposited on the absorber 6 and patterned, and the unwanted portions of the absorber are etched off. Following that, a photolithography process is performed to transfer a desired pattern onto the absorber 6. Then, referring to FIG. 2D, after the removal of the patterned photoresist 8, an anti-reflection layer 10 is deposited on the patterned absorber 6, and a mask for x-ray is completed. The anti-reflection layer 10 is used to prevent the transfer of patterns caused by exposure to secondary electrons due to excessive x-ray energy.
And then, referring to FIG. 2E, an etching process is performed on the middle portion of the lower membrane 4 and the into the silicon wafer 2. The etching process, such as one using KOH, removes the middle portion of the silicon wafer 2, and stops as it reaches the upper membrane 3 to accomplish the fabrication of an x-ray mask.
The previously mentioned drawbacks of a conventional x-ray mask also happen to the second conventional x-ray mask during the selective etching process and after several x-ray exposures.